1. Technical Field
Embodiments of the present invention relate to a fixing device to fix an image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Image forming apparatuses are used as, for example, copiers, printers, facsimile machines, and multi-functional devices having at least one of the foregoing capabilities. As one type of image forming apparatus, electrophotographic image forming apparatuses are known. Such electrophotographic image forming apparatuses may have a fixing device to fix a toner image on a sheet of paper serving as a recording medium. Such a fixing device includes, for example, a fixing rotary body heated by a heating member and an opposed member to contact the fixing rotary body. The fixing rotary body and the opposed member contact each other to form a nipping portion. When a sheet having a toner image passes through the nipping portion, toner is fused under the heat of the fixing rotary body and fixed on the sheet.
Typically, such a heating member heats the fixing rotary body over an entire width of the sheet passing the nipping portion. As a result, the entire sheet is heated by the fixing rotary body. However, when an image is placed on only a portion of the sheet, heat energy is wasted in a non-image area, i.e., an area having no image.
To reduce such waste of heat energy in the non-image area, a fixing device is proposed to adjust a heating area in accordance with an image on a recording medium to heat a portion to be fixed without heating a portion not necessary to be fixed (for example, JP-H06-095540-A, JP-2001-343860-A, and JP-2005-181946-A).
For the above-described configuration of adjusting the heating area in accordance with the distribution of an image area and a non-image area in a desired image, in particular, when both the image area and the non-image area exist in the width direction of a sheet, a temperature difference may occur in a longitudinal direction of the fixing rotary body and the opposed member. The temperature difference in a surface of the opposed member changes the diameter size of the opposed member due to thermal expansion difference. As a result, a difference in conveyance speed of the sheet occurs in the longitudinal direction of the opposed member, thus resulting in a conveyance error (e.g., wrinkles in the sheet). In addition, the temperature difference in a surface of the fixing rotary body causes thermal stress due to a difference in thermal expansion amount. As a result, deformation called kink may occur, thus reducing image quality. Such failures may be prominent when the fixing rotary body is formed of a flexible thin member, such as belt or film.